1. Field of the Invention
The present invention relates to a fluorescent lamp capable of reducing the occurrence of blackening, is smaller in the rate of lowering luminous flux and rapid rising of luminous flux at starting stage of lighting.
2. Description of the Related Art
The fluorescent lamp is widely used as light sources for common illuminations, office automation machines, and light-emitting elements of the giant screen color display apparatus and also as back lights for liquid crystal displays. The reason why it is so widely used in various fields resides in that the electric power supplied can be converted into radiation with extremely high efficiency.
This fluorescent lamp is arranged so that the filling gas including mercury and one or two or more rare gases are filled in a glass tube on whose inner face a fluorescent layer is coated and ,that positive column discharge is generated in the filling gas.
The positive column discharge can be usually kept generated by supplying electric energy to the filling gas between two electrodes. Mainly ultraviolet rays are generated by this discharge and most of them have wavelengths of 185 nm and 254 nm. The intensity ratio of the 185 nm light to 254 nm light is usually in a range of 0.2-0.4. These ultraviolet rays are converted into radiation which has longer wavelengths by the fluorescent layer on the inner face of the glass tube. These wavelengths depend upon the kind of phosphor particles contained in the phosphor layer and they range from those of near ultraviolet rays to those of visible rays and even those of near infrared lights. The most popular low fluorescent lamp is of the 40 W type shaped like a straight glass tube having a length of 1200 mm and an inner diameter of about 37 mm. The inner wall of this lamp is about 300 W/m.sup.2. Note that "inner wall load" is defined a consumptive power per unit area of the inner face.
The glass tube can be variously shaped like a circle, a letter U and a saddle in addition to like the straight line. As the fluorescent lamps have been made smaller and smaller in size, most of their glass tubes have been shaped in various complicated forms.
As the color rendering properties of the fluorescent lamp have been deemed as being more and more important, the three component type white-emitting fluorescent lamp in which blue, green and red emitting phosphors each having a relatively narrow band emission spectrum distribution are used has become more and more popular. In the case of this three component type fluorescent lamp, the characteristics of the green emitting phosphor is important because the luminous ratio of the green component to the white luminous flux (or output) of the lamp is the highest among the three color components. Rare-earth aluminate-silicate-phosphate phosphor, rare-earth borate-silicate-phosphate phosphor, rare-earth orthophospate phosphor, rare-earth aluminate borate phosphor and the like, each is activated by cerium and terbium, are well known as the green emitting phosphor.
As the fluorescent lamp has been made smaller and smaller in size as described above, the inner wall load of its tube increased. Some fluorescent lamps each having a relatively high inner wall load are disclosed in German Patent Disclosure 2109898 and others. However, these lamps of a relatively high inner wall load have such drawbacks that their lamp efficiency is lower, that the rate of their lowering luminous flux is higher, that the occurrence of their blackening is faster, and that the rising of their luminous flux is slower, as compared with the common mercury lamp.
Japanese Patent Disclosure Sho 54-42874 discloses a lamp wherein phosphor particles whose positive ions have the elemental electronegativity values lower than 1.4 are used to form the phosphor layer which has a low luminous flux lowering rate. However, the single measure disclosed in this reference is not enough to prevent luminous flux from being lowered and the lamp from being blackened.
Blackening of the fluorescent lamp has been considered to be caused by the following factors. Firstly the phosphor itself is deteriorated. The reasons are supposed to reside in that the color center is formed in the phosphor by vacuum ultraviolet rays produced by the positive column discharge and that the surface structure of the phosphor layer is changed by the colliding of Hg ions at the surface of the phosphor layer. Secondly, the glass tube is black ended or colored by impurities and reaction products in it. This is supposed to reside in that vaporized fragment and cathode matters, compounds caused by residues of organic binders, and/or mercury and mercury compounds contained in the filling gas adhere to the phosphor layer.
Those extents to which the lamp glass tubes are blackened are sometimes different from one another and the qualities of the lamps produced are not the same in the case of the conventional fluorescent lamp which has a relatively high inner wall load, depending upon the kinds of phosphor particles (or fluorescent matters) used even though the lamps are manufactured according to the same process and the lamps thus manufactured are the same in structure and also depending upon the manufacturing lots even though the phosphor particles used are of the same kind. Particularly in the case of the green emitting rare-earth phosphor, the blackening phenomenon is more liable to be caused by contaminations originating from the adhesion of mercury and its compounds, as compared with the red emitting phosphor. In the case of the three components type white emitting fluorescent lamp whose luminous flux depends severely on the luminous efficiency of the green emitting phosphor used, therefore, the above-mentioned drawbacks become more remarkable.
As described above, the conventional fluorescent lamp which has a relatively high inner wall load has advantages of being made small and compact in size, but it has such drawbacks that its luminous efficiency is lower, that its blackening is liable to be caused earlier, that its luminous flux lowering rate is higher, and that the rising of its luminous flux is slower. In addition, these drawbacks are not caused in the same way and it is difficult for the lamps thus produced to have the same quality.
The reason why the rising of luminous flux is slow is supposed to be caused by the mercury adsorption to the phosphor. Particularly when the inner wall load of the lamp becomes high, the probability that mercury ions are re-bonded with electrons to become mercury atoms at the surface of the phosphor particles becomes high as electric current density is increased. The mercury atoms are liable to be adsorbed onto the phosphor layer when the lamp is turned off as well as when it is turned on. When it is again turned on, this mercury adsorbed is firstly desorbed. Mercury cohering to the coolest part of the lamp so then gradually vaporized and luminous flux rises with increase in the vapor pressure of the mercury in the glass tube. FIG. 1 shows the relation between relative luminous flux and lighting time in the case of the conventional fluorescent lamps, wherein a curve (A) represents the rising of luminous flux in the case of the fluorescent lamp of 40 W which has a relatively low inner tube wall load and which is shaped like a straight tube and another curve (B) that of luminous flux in the case of the fluorescent lamp in which the three component bands emitting phosphor is used and whose inner wall load is larger than 500 W/m.sup.2. In FIG. 1, a part (x) denotes the time at which mercury adsorbed on the phosphor layer is adsorbed and another part (y) the time at which mercury cohering to the coolest part of the lamp is gradually vaporized.